直齿轮“渗碳-温挤”渗碳层分布与性能研究

齿轮广泛用于各种机械设备和仪器仪表中,是机械传动的基本零件。近年来,随着汽车工业的发展,特别是轿车生产,对齿轮的精度及力学性能的要求越来越高,齿轮正朝着高精度、低噪声、高承载、高速度、轻量化及长寿命的方向发展。其中,采用硬齿面齿轮是提高齿轮强度及承载能力的有效途径。目前,硬齿面直齿轮普遍采用“机加-渗碳-热处理”的工艺加工,材料利用率不高,尤其是金属流线被切断,而且成形后渗碳处理使渗碳层晶粒粗大、渗碳层厚度分布不合理,造成齿轮强度与疲劳寿命的降低。而直齿轮“渗碳-温挤”技术将坯料先做渗碳处理,再温精密塑性成形为齿轮,具有材料利用率高、生产率高及晶粒细化等优点。将文讨论“渗碳-温挤”直齿轮工艺的渗碳层分布与齿轮性能关系。     

采用稀土共渗技术樨决汽车齿轮渗碳存在的问题（下）

三、稀土渗碳工艺的选择 按目前生产上渗碳齿轮的模数与渗碳层深度可将其按附表所列进行分类，并根据稀土渗碳特点：提高渗速15％～30％，降低渗碳温度40～60℃，节能15％～40％，减少变形40％～60％，明显改善金相组织，大幅提高工件使用性能等，与齿轮的模数及工作条件相结合，进行统一整合，分别采用不同工艺方法进行稀土渗碳，以最终综合技术经济指标及有针对性地解决生产问题作为衡量标准，作为选择稀土渗碳工艺的依据，以获取最大技术经济效果。     

齿轮渗碳工艺的改善

一、前言较为重要的齿轮多用渗碳钢制造,并经渗碳、淬火和回火达到技术要求。齿轮各部位具有显著不同的曲率半径,因此渗碳后各部位的表面含碳量及渗层厚度和组织有较大差异,齿顶角处易出现碳化物聚集呈网状或半网状分布;齿根处渗层厚度要浅些。某些工厂生产的齿轮金相组织还不能稳定地达到技术要求,渗碳后在缓冷中出现脱碳现象,使淬火后硬度偏低,耐磨性差,易被擦伤且疲劳强度低,使用寿命短。齿轮的形状复杂,渗碳淬火后往往出现较大的变形,从而影响齿轮的传动精度、接触精度和传动平稳     

汽车齿轮表面脱碳的影响及工艺改进

本文对汽车齿轮热处理渗碳件轮齿表面出现较严重的脱碳现象进行检查分析。检测结果表明,组织中大量条状及块状铁素体,使硬度分布不均匀并形成软点,齿轮啮合的承载能力降低,而且影响齿轮表面的耐磨性。通过对20Cr Mn Ti低碳合金渗碳钢齿轮从坯料的齿坯成形、中间退火处理到最后渗碳淬火的整个热处理过程,进行检查和工艺改进。     

渗碳淬火齿轮渗层质量评价综述

渗碳淬火工艺用来改善齿轮表面接触疲劳强度、弯曲疲劳强度、表面耐磨性等综合性能,广泛应用在齿轮热处理工艺中。严格控制渗层质量是获得齿轮最佳综合性能的关键。介绍了渗层中有效硬化层深度、碳化物、残余奥氏体等指标的评价要求。并结合国内外对渗碳层质量控制规范的现状和相关标准,对我国渗层质量评价规范提出建议。     

渗碳层深度不合格的原因与防止方法

从多年的热处理实践中体会到齿轮在渗碳过程中。由于设备、材料、工艺和操作者等诸多方面的原因,会出现各种各样的缺陷。如渗碳层深度不合格会影响产品质量和生产效率。因此,有必要弄清楚缺陷产生的原因与防止方法。     

20MnCr5钢齿轮表面渗碳层的显微组织

在930℃下对20MnCr5钢齿轮进行常规真空渗碳热处理,利用光学显微镜、扫描电镜、透射电镜、电子探针和显微硬度计等研究了渗碳层的显微组织、元素分布和显微硬度。结果表明：渗碳层表层组织由高碳针状马氏体与残余奥氏体组成,随距表面距离增大,针状马氏体向板条马氏体转变;渗碳层中还析出了条状富铬碳化物和球状富锰碳化物,碳化物呈弥散分布;渗碳层表面硬度为860 HV,远高于基体,随距表面距离增大,渗碳层硬度下降。     

17CrNiMo6钢齿轮成品淬硬层深与渗碳层深探讨

正新型合金齿轮渗碳钢17CrNiMo6钢的应用越来越广,但是其许多基本性能如工艺特性资料缺乏。本文结合生产实际需要,对某外协厂家17CrNiMo6钢的热处理工艺渗碳层深确定进行研究,为合金齿轮渗碳热处理冷热工艺衔接和渗碳层深确定提供理论参考。1.试验用材料及方法(1)材料选用的材料为17CrNiMo6钢,其主要化学成     

重载齿轮渗碳的计算机控制

用微型计算机实现多因素气相碳势精确控制,其精度不受渗剂种类的影响,适应性强。另一方面,为了保证重载齿轮获得理想的渗层浓度分布,本文在深入研究渗碳的数学模型基础上,发展了将优化渗碳工艺的电子计算机辅助设计与实时控制结合在一起的“动态控制”技术。这种新技术已应用于生产,取得提高渗碳齿轮质量,重现性好和显著节能的效果。     

大模数齿轮花键轴的渗碳淬火应用

齿轮渗碳淬火提高强度和耐磨性的应用已相当普及,但该工艺的实际应用也仅限于中、小模数(m<8)齿轮。对大模数齿轮,要求渗碳层深度大,渗碳周期长,直接淬火工艺操作不易掌握,其实际应用不多。     

On the stress distribution in prestressed extrusion dies under non-uniform distribution of internal pressure

Cold extrusion die stress distribution is normally calculated on the assumption that there is a uniform distribution of internal pressure, e.g. by application of the Lamé equations. With FE-analysis this assumption can be overcome by the introduction of arbitrary boundary conditions. However, as little information existed about realistic distribution of radial and axial stresses in container and die this possibility was not used very much. Only after a recent investigation of Bay [1] into friction and pressure distribution in forward extrusion an FE-calculation of stresses in a prestressed extrusion die under non-uniform distribution of radial pressure seemed to be promising. The results show very clearly that no negative effects on the stress state—e.g. increase of stress peaks—may be expected in case of combined continuous decrease of radial pressure in the container and discontinuous increase at the die entry.     

A study on reheating characteristics for thixo die casting process with electromagnetic stirring and extruded aluminum alloys and their mechanical properties

In this paper, the reheating process and mechanical properties of thixo die cast A356 alloy, using electromagnetic stirring (EMS) and extrusion before thixo die casting, were discussed. The EMS is used mainly to manufacture raw materials with a globular microstructure. If relevant reheating is carried out, products with excellent mechanical properties can be manufactured by EMS. Contrarily, extruded material has a fine dendrite microstructure and has rarely been assumed to be suitable for the thixo die casting process. This study demonstrated that the microstructure of extruded material could be globularized and applied to the thixo die casting process only with the relevant reheating parameters. This applicability was estimated through image analysis obtaining the solid fraction, the mean equivalent diameter, and roundness under various reheating conditions. After thixo die casting experiments using three materials, which were EMS, low-extrusion material (LER), and high-extrusion ratio materials (HER), mechanical properties were comparatively analyzed. EMS and HER materials show better mechanical properties than LER.     

A computational study of die geometry and processing conditions effects on equal channel angular extrusion of a polymer

Equal channel angular extrusion (ECAE) is an efficient process to obtain enhanced microstructures via super-plastic deformation. In view of its optimisation, it is of prime importance to assess the relationships between processing conditions and material flow. More precisely, detailed knowledge of the plastic strain distribution in the extruded material in relation to the ECAE processing variables is required. The key parameters of the ECAE process are primarily die geometry, ram speed, extrusion temperature, use of back-pressure, number of extrusion sequences and processing route (e.g. rotation of the sample between successive passes). A numerical investigation was achieved to check out the influence of these parameters on the homogeneity of plastic strain distribution in the case of a conventional thermoplastic polymer. Material parameters of a phenomenological elastic viscoplastic model were deduced from compressive deformation tests at different temperatures and strain rates on high-density polyethylene (HDPE). Recommendations on tool geometry and processing conditions can then be provided, according to the numerical results.It was found that optimum ECAE die geometry is strongly material dependent. The application of a back-pressure significantly contributes to reduce the corner gap and consequently promotes the homogeneity of the plastic strain field. A slight sensitivity of plastic strain to ram speed and friction conditions was pointed out. The extrusion temperature strongly influences the magnitude of the plastic strain and has a slight effect on its homogeneity. The number of passes has a significant effect on the magnitude of the plastic strain but has a negligible influence beyond a certain temperature. The extruded material reaches a stationary strain state after few passes. The homogeneity of the plastic strain field is strongly affected by the processing route.     

Microstructure analysis of aluminum extrusion: grain size distribution in AA6060, AA6082 and AA7075 alloys

Microstructure and material flow of aluminum alloys have a significant influence on the mechanical properties and surface quality. In extrusion of aluminum billets at high temperatures the microstructure is dependent on the alloy and the forming and temperature history. A prediction of grain size and precipitation is of increasing importance in order to design the process by adjustment of parameters such as punch speed, temperatures, and quenching. To give references for microstructure prediction based on material flow, and with it strain and strain rate history, this paper deals with the microstructure during the extrusion process of AA6060, AA6082, and AA7075 alloys. Billets have been partly extruded to axisymmetric round profiles and the microstructure of the press rests consisting of the billet rests in container and die has been considered. Furthermore, these rests have been analyzed to show the material flow, dynamic and static recrystallization based on macro etchings and visible microstructure under different conditions, e.g. as in the area of high strain rate near the container wall, or in dead zones [1]. To allow an accurate simulation of the extrusion process, punch force and temperature conditions during the tests have been measured and are presented in this paper, too.     

Effects of nanocrystallized layer on the tribological properties of micro-arc oxidation coatings on 2618 aluminum alloy under high temperatures

As one of the incremental bulk metal forming, the main plastic metal flow of axial closed die rolling (ACDR) process consists of the axial compression and circumferential torsion. The key difference between the ACDR and forging processes should be concentrating on the plastic deformation zone (PDZ) evolution, especially on the condition of thermal deformation. Therefore, the verified FEM and corresponding experiment have been proposed to find out the systematic rule of PDZ. The thermal parameter distribution of the workpiece has been explored to obtain the PDZ evolution at different deformation extent. PDZ evolution mode has also been proposed, and the distribution of microstructure/hardness is given to the evidence of PDZ mode. The results show that the PDZ spreads from the contact area of the upper surface of the lower/side surface in ACDR process. Multiple thermal parameter evolution in ACDR process could be found through the analysis of microstructure morphology. Also, microstructure orientation could be the fundament of PDZ evolution. The cooling rate and the periodic evaluation of thermal parameters should be the main factors for the microstructure evolution, including the distribution of lamellar α and globular α. Then, the extent and range of PDZ could be defined through the observation and analysis of hardness evolution.     

Improvement of mechanical properties of Al6061 extrudate by die cooling with N2 gas during hot extrusion

In this study, the mechanical properties of the extrudate are improved by correcting the defects such as abnormal grain growth (AGG). Die cooling with N₂ gas is conducted to control the extrusion temperature during the hot extrusion of Al6061 in order to obtain fine grains of the extrudate. Computational fluid dynamics (CFD) is conducted to evaluate the effect of die cooling with N₂ gas and to determine the relationship between AGG and the extrusion temperature. The optimal cooling channel with a high cooling effect is designed by using the design of experiment (DOE) method, and thermo-mechanical analysis is performed to predict the extrusion temperature. A hot extrusion experiment is also carried out to measure the extrusion temperature and to observe the microstructure of the extrudate. The extrusion temperature predicted from the thermo mechanical analysis is found to be in good agreement with the measured extrusion temperature. The process condition required to reduce the AGG is determined on the basis of the distribution of the evolved microstructure in the hot extrusion experiment. The results confirm the relationship between the distribution of AGG and the mechanical properties of the extrudate. It is confirmed that the mechanical properties of Al6061 can be improved by the application of die cooling with N₂ gas during the hot extrusion process.     

Microstructure and properties of surfacing layers of dies manufactured by bimetal-gradient-layer surfacing technology before and after service

The purpose of this article is to investigate the microstructure and mechanical properties of surfacing layers (wear layer and transition layer) of a hot forging die manufactured by the bimetal-gradient-layer surfacing method, which is based on ZG29MnMoNi cast steel before and after forged 5761 parts on a 63MN hot die forging press. The finite element model of a die was established. Subsequently, a simulation was conducted to analyze the temperature field of the die and its cycle features under working conditions. Microstructure and mechanical property were measured. Results indicated that the microstructure of the wear layer mainly consists of temper sorbite, ferrite and carbides. The transition layer before and after service is mainly composed of both temper sobrite, lower bainite, and a small amount of temper martensite. The mechanical properties of wear and transition layers declined significantly after service. The tensile strength, yield strength, reduction of area, and elongation of wear layer declined by 41.6, 32.5, 28.3, and 24.5 %, respectively. With those indexes the transition layer decreased by 36.6, 34, 24.4, and 19.8 %, respectively. Microhardness and impact energy of wear and transition layers have showed a decrease of 17, 6 % and 51.2, 32.6 %, respectively. The impact fracture mode of both wear and transition layers is typically intergranular fracture after service. As a conclusion, it was determined that the service process sufficiently influenced the mechanical properties of the surfacing layers.     

Influence of friction during forming processes—a study using a numerical simulation technique

Friction has an important influence in metal forming operations, as it contributes to the success or otherwise of the process. In the present investigation, the effect of friction on metal forming was studied by simulating compression tests on cylindrical Al-Mg alloy using the finite element method (FEM) technique. Three kinds of compression tests were considered wherein a constant coefficient of friction was employed at the upper die–work-piece interface. However, the coefficient of friction between the lower die–work-piece interfaces was varied in the tests. The simulation results showed that a difference in metal flow occurs near the interfaces owing to the differences in the coefficient of friction. It was concluded that the variations in the coefficient of friction between the dies and the work-piece directly affect the stress distribution and shape of the work-piece, having implications on the microstructure of the material being processed.     

The effect of billets extruded by a curved and flat-face die on the semisolid characteristics and tensile properties of thixoformed products

In this study, A356 aluminum billets in different extruded states are used as feedstock for the thixoforming. The extrusion billets were fabricated by a hot extrusion process through a flat-face and a curved die. After the induction reheating of the extrusion billets into a semisolid state, the microstructural evolution was thoroughly investigated. For the extrusion alloy by the flat-face die, there was a large variation in the average grain size (20 %) and the mean roundness (17 %) of equiaxed α-Al grains. This, together with evidence of elongated grains in the interior regions of the billet, indicated that a noticeably nonuniform globular microstructure had been obtained. In contrast, for the extrusion alloy through the curved die, the obtained globular microstructure was more uniform. There were slight variations of 5 % and 7 % in the average grain size and the mean roundness, respectively. By using the extrusion billets, some parts fabricated via the thixoforming process those underwent T6 heat treatment. The tensile test results for the fabricated parts showed that when the extrusion billet through the conventional flat-face die was used as the feedstock, there was a large scattering in the tensile properties throughout the part. In contrast, when the extruded billet through the curved die was used as the feedstock, limited variation was observed in the tensile properties.     

Analysis and porthole die design for a multi-hole extrusion process of a hollow,thin-walled aluminum profile

The appropriate die design for multi-hole extrusion is still a challenging task because of the complicated circumstances and large material deformation during extrusion process. In the present study, the material flow during multi-hole extrusion process for producing a hollow and thin-walled profile was revealed by means of numerical simulation based on the Arbitrary Lagrangian Eulerian (ALE) method. The effects of eccentricity ratio, shape of the second-step welding chamber, and uneven bearing length on the exit velocity distribution of extrudate were synthetically investigated, and a two-hole porthole die was designed accordingly. The exit velocity and temperature on the extrudate in this optimized die were analyzed and compared with the initial die, and it was found that both of them exhibit better uniformity, which is beneficial for the enhancement of product quality. Through performing the current work, a logical and effective route for designing multi-hole porthole die was proposed as the guidance for die designers.